Remember to turn off the lights

This is not really a profound case. It's just that I live with 5 other people and guys tend to leave the light on everywhere. We have two bathrooms, kitchen, corridor. But also they have the lights on in their rooms when they are out. It's a huge waste! I don't want to be 'that bitchy roommate' so this is a fun way for them to remember about turning the lights off :D.